1. Field of the Invention
The present invention relates to a slip control system for a motor vehicle and, in particular, to a slip control system which is able to improve the stability of a motor vehicle when the vehicle undergoes a steering operation.
2. Description of Related Art
There have been proposed slip control systems for motor vehicles such as those shown in Japanese Patent Laid-Open Publication No. 58-16948, No. 57-22948, No. 2-231836, No. 64-94029, and the like. When an excessive slip is caused between driving wheels and road surfaces during starting and/or running operations, the slip control system decreases the excessive slip under a predetermined value to obtain driving forces which are transmitted to the road surfaces from the driving wheels. Therefore, not only starting and accelerating abilities but also running and driving stabilities are improved.
The slip control system may include an engine control system which decreases a degree of opening of a throttle valve, delays an ignition timing, and cuts fuel to the engine so as to decrease an excessive engine output. The slip control system also may include a brake control system which increases brake fluid pressure so as to transitionally increase a braking force of driving wheels. When the vehicle is running on a low friction coefficient road surface, an excessive depression of the acceleration pedal causes an excessive slip of the driving wheels. The engine control system and/or brake control system decreases the driving torque transmitted to the driving wheels so as to decrease the excessive slip under a predetermined desired value and obtain driving forces which are transmitted to the road surfaces from the driving wheels.
The behavior characteristic of the vehicle in the slip condition of the driving wheels when the vehicle is running straight is different from that when the vehicle is steered. When the vehicle is steered, a relatively large change in the behavior of the driving wheels is caused by a relatively small slip and, therefore, the vehicle is unstable.
However, since the above-mentioned slip control system controls the driving torque transmitted to the driving wheels in accordance with substantially the same desired slip ratio when the vehicle is running straight and when it is steered, the vehicle is not able to correspond properly to the behavior change of the driving wheels in the steering operation, so that the vehicle is unstable.